Conventional solid composite propellants have binders which utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to cross-linked elastomers. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the "pot life". Disposal of a cast cross-linked propellant composition is difficult, except by burning, which poses environmental problems.
As an alternative to cross-linked elastomer binders, U.S. Pat Nos. 4,361,526 proposes to use a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystalline structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. In order to prepare a propellant composition using the copolymer, the copolymer is dissolved in an organic solvent, such as toluene, and the solids and other propellant formulations components are added. The solvent is then evaporated, leaving a rubbery solid which may be divided into pellents suitable for casting or other processing.
A disadvantage of formulating a propellant using a thermoplastic elastomeric binder which must be dissolved in a solvent is that the propellant grain cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to solvent removal and recovery. Organic solvents, such as toluene, present certain hazards both to the immediate work area and to the larger environment, necessitating various precautions to be taken with respect to processing such propellant formulations.
It has also been proposed to produce thermoplastic elastomeric propellants in which the solid particulates and thermoplastic elastomer are fused in a high-shear mixer or extruded and the fused mixture poured or extruded into a casing or mold. There are important disadvantages with such techniques. Because of the high-solids loading of propellant formulations, viscosities tend to be quite high, making mixing difficult and, in many cases, impossible on a large scale. Localized overheating in high-shear apparatus may cause instability in a high-energy formulation. As will be described in greater detail hereinafter, casting techniques which rely on flow, e.g., pouring or extrusion, generally result in an undesirable anisotropic orientation of particulates.
There exists a need for improved processes for producing propellant grains with thermoplastic elastomeric binders, and particularly for processes which can be scaled up to produce even the largest rocket motors.